The need to reduce the space requirement of antenna systems and the search for improved radiation performance and lower manufacturing costs are leading the designers of these systems to develop novel materials.
Recent years have seen a major interest in metamaterials. The notion of metamaterials is well known and is discussed for example in J. B. Pendry, A. J. Holden, D. J. Robbins, and W. J. Stewart, “Magnetism from conductors and enhanced nonlinear phenomena,” IEEE Trans. Microw. Theory Tech., vol. 47, no. 11, pp. 2075-2084, 1999.
It may simply be recalled that metamaterials are by definition metal-dielectric composite media. They are periodic structures whose constituent elements are metal inclusions of very small dimensions relative to the wavelength (<λ/10).
There are many types of metamaterial structures.
Electric metamaterials are metamaterials which have electric behavior and are liable to show negative permittivity (∈) in a given frequency spectrum. The best-known electrical metamaterials are those formed by an array of metal rods.
Magnetic metamaterials are metamaterials which have magnetic behavior and are liable to show negative permeability (μ) in a given frequency spectrum. The best-known magnetic metamaterials are those formed by an array of square or circular split-ring resonators (SRR).
The left-handed materials (LHM) are metamaterials liable to show permittivity (∈) and permeability (μ) that are simultaneously negative in a given frequency spectrum. The best-known left-handed materials are those formed by the combination of an array of metal rods and an array of split-ring resonators. With such left-handed materials, it is possible to obtain wholly unexpected propagation phenomena such as opposite phase and group speeds, inverse Doppler effects, negative refraction indices, etc.
In the field of electromagnetic waves, it has been proposed to use left-handed materials of this kind as antenna radomes.
FIG. 1 illustrates an example of an antenna system comprising a radome made of left-handed material based on split-ring resonators and conductive strips. For reasons of clarity, only half of the antenna system is shown in FIG. 1.
The antenna system 10 comprises:                an antenna 110 comprising:                    a carrier structure 11 comprising a ground 12 (or ground plane) and a layer 13 of dielectric material and/or magnetic material placed on ground 12;            a radiating element 14 placed on the carrier structure 11, and                        a radome 15.        
The radome 15 extends above the antenna 110. The radome 15 is separated from the antenna 110 by a volume 16 constituted for example by air or dielectric and/or magnetic material.
The radome 15 comprises a structure of left-handed material. The structure of left-handed material comprises a plurality of elementary blocks 17 arranged in rows and columns in a matrix. Each elementary block 17 comprises a split-ring resonator and a conductive strip.
FIG. 2 illustrates a possible example of an elementary block of left-handed material based on split-ring resonators and conductive strips.
The elementary block of left-handed material 20 comprises a first support 21 made of a dielectric material comprising an upper face 22 on which there is placed a split-ring resonator 24 and a lower face 23 on which there is placed a first linear metal strip 25. The elementary block 20 comprises a second support 26 made of dielectric material comprising a lower face 27 on which there is placed a second linear metal strip 28. The two supports 21 and 26 are separated by an air layer 29.
The split-ring resonator 24 comprises an inner slotted square 241 and an outer slotted square 242. By way of an example, for an X band operation (8.2 GHz to 12.4 GHz), the width of the slot of each slotted square is about 0.3 mm. The width of the different metal tracks (split-ring resonator and metal strips) is about 0.3 mm. The spacing between the inner slotted square 241 and outer slotted square 242 is about 0.3 mm. The volume of an elementary block 20 is about 3.3×3.3×4.5 mm3 and the periodicity of the metamaterial structure is about 3.63 mm in the plane and 4.5 mm in depth.
The radome 15 plays the role of a device for diffracting electromagnetic waves and increases the directivity and the gain of the antenna 101 while at the same time reducing the minor lobe and rear radiation levels. This is described especially in detail in the document Shah Nawaz Burokur, Mohamed Latrach, and Serge Toutain “Theoretical Investigation of a Circular Patch Antenna in the Presence of a Left-Handed Medium”, IEEE Trans. Antennas and Wireless Propagation Letters, Vol. 4, page 183-186, 2005.
However, this left-handed material structure based on split-ring resonators and conductive strips has several drawbacks.
One of the drawbacks of this structure of left-handed material based on split-ring resonators and conductive strips is that it works only with linear polarization antennas. In other words, this structure cannot be used in circular polarization.
Besides, it is desirable that the structure of left-handed material (forming the antenna radome) should be simple to make and should have the lowest possible cost.
Several solutions have been proposed in this respect.
One known solution is described in the US patent document 2010/0097281. This solution uses a left-handed material based on S-shaped resonators.
FIG. 3 illustrates an example of an elementary block of left-handed material based on S-shaped resonators (placed on one face of a support made of dielectric material) and inverse S-shaped resonators (placed on the other face of the support). The particular feature of this type of resonator 30 is that it has dual resonance, magnetic and electric, without requiring the implementation of small-sized slots and an additional array of metal rods.
Thus, a structure made of left-handed material based on S-shaped resonators has great simplicity of manufacture. However, it has the drawback of not working when the polarization of the antenna is circular.